The various functions that are required of electrophotographic photoconductors include, ordinarily, a function of holding surface charge, in the dark, a function of generating charge through reception of light, and a function of transporting charge likewise through reception of light. Such photoconductors include so-called single-layer-type photoconductors, having a single photosensitive layer, wherein these functions are combined in one layer, and so-called multilayer-type photoconductors having a photosensitive layer that is a stack of layers functionally separated into a layer that contributes mainly to charge generation, and a layer that contributes to holding surface charge, in the dark, and to charge transport during light reception.
For instance, the Carlson method is used in image formation by an electrophotographic method that utilizes such electrophotographic photoconductors. Image formation according to this scheme involves charging a photoconductor in the dark, forming an electrostatic image, such as text or pictures of an original, on the charged photoconductor surface, developing the formed electrostatic image by means of toner, and transferring and fixing the developed toner image onto a support such as paper. The photoconductor after toner image transfer has residual toner and charge removed therefrom and is re-used.
Materials used in the above-described electrophotographic photoconductor include inorganic photoconductive materials such as selenium, selenium alloys, zinc oxide, and cadmium sulfide, dispersed in a resin binder; organic photoconductive materials such as poly-N-vinyl carbazole, 9,10-anthracenediol polyester, pyrazoline, hydrazone, stilbene, butadiene, benzidine, phthalocyanine or bisazo compounds, dispersed in a resin binder, or materials resulting from vacuum vapor deposition or sublimation of the foregoing.
Electrophotographic printing devices are required to possess ever higher durability and sensitivity, and faster responses, to cope with, for instance, increases in the number of copies to be printed in a networked office, and with the rapid development of lightweight electrophotographic printing machines. These devices, moreover, are held to strict requirements in terms of exhibiting low impact from gases, such as ozone and NOx that are generated in the device, as well as little fluctuation in image characteristic arising from variations in the usage environment (room temperature, humidity).
At present, however, conventional photoconductors do not necessarily satisfy in full the characteristics that are required from them, and are still problematic as regards the points below.
For instance, wear resistance is bedeviled by the following problems. With the introduction of tandem development and other schemes, high-speed printing machines have gained popularity in recent years, both in printers and copiers for monochrome printing as well as in models for color printing. Color printing, in particular, requires high resolution, and the positional precision of images is thus a major concern among the required specifications. As the printed copies pile up, the surface of the photoconductor is abraded by friction against paper, rollers, blades and the like. When the degree of such wear is significant, it becomes difficult to print images that boast high resolution and high image positional precision. Various approaches have been adopted in order to enhance wear resistance, but cannot be said to be fully perfected yet.
Ozone is well known as one of the gases that are generated in these devices. Ozone is generated by chargers and roller chargers that elicit corona discharge. It is deemed that the organic substances that make up the photoconductor are oxidized, and the original structure of the substances breaks down when the photoconductor is exposed to the ozone that remains or is retained within the device, and the photoconductor characteristic are significantly impaired. Moreover, it is found that ozone oxidizes the nitrogen in air into NOx, and that this NOx alters the organic substances that make up the photoconductor.
It is deemed that characteristic deterioration elicited by such gases extends not only to the outermost layer of the photoconductor, but that adverse effects arise also when the gas flows into the interior of a photosensitive layer. It is found that the outermost layer itself of the photoconductor is scraped off, though the amount of scraping varies, on account of friction with the above-described various members. When a harmful gas flows into the interior of the photosensitive layer, the organic substances in the photosensitive layer may undergo structural breakdown. Suppressing the inflow of such harmful gas is thus an issue to be addressed. In tandem-type color electrophotographic devices that rely on a plurality of photoconductors, in particular, variation in color occur as a result of differences in the degree of influence of the gas, depending on, for instance, the position at which drums are disposed in the device. Such variations are deemed to constitute an impediment to forming adequate images. Therefore, it is found that characteristic deterioration caused by gas is a particularly important issue in tandem-type color electrophotographic devices.
For instance, Patent Document 1 and Patent Document 2 disclose the feature of using an antioxidant compound, such as a hindered phenol compound, or a phosphorus-based compound, a sulfur-based compound, an amine-based compound, a hindered amine-based compound or the like. Patent Document 3 proposes a technology that involves using a carbonyl compound, and Patent Document 4 proposes a technology that involves using a benzoate-based or salicylate-based compound. Techniques proposed in order to enhance gas resistance include using an additive such as biphenyl or the like and using a specific polycarbonate resin, in Patent Document 5; combining a specific amine compound with a polyarylate resin, in Patent Document 6; and combining a polyarylate resin and a compound having a specific absorbance, in Patent Document 7. However, these techniques fail to afford a photoconductor that exhibits sufficient gas resistance. Even if the photoconductor did exhibit satisfactory gas resistance, the technologies do not address the issue of enhancing the wear resistance of the photoconductor. Moreover, satisfactory results are not yet forthcoming as regards other characteristics (for instance, image memory and potential stability in durability printing).
Patent Document 8 discloses the feature of prescribing the oxygen permeability coefficient of a surface layer to be no greater than a predetermined value, under a combined condition whereby a charge transport layer has a specific charge mobility, so that, as a result, it becomes possible to curb the influence exerted on a photoconductor by the gas that is generated around a charger. Patent Document 9 indicates that wear resistance and gas resistance can be enhanced by prescribing the water vapor permeability of a photosensitive layer to be no greater than a predetermined value. In this technology, however, the desired effect cannot be achieved unless a specific polymer charge transport substance is used. Thus, the mobility, structure and so forth of the charge transport substance are restricted, and hence the technology failed to meet in full various requirements as regards electrical characteristics.
Patent Document 10 indicates that a single-layer-type electrophotographic photoconductor having excellent gas resistance can be provided by using a specific diester compound, having a melting point not higher than 40° C., in a photosensitive layer. In this case, however, the substance of low melting point that is added into the layer is in contact, for a prolonged time, with parts of the device main body or cartridge in which the photoconductor, having the substance added thereinto, is used, and hence the compound may become smeared into the other part with which the compound is in contact, giving rise to so-called bleeding, which translates into defects on the image. A sufficient effect failed to be elicited here as well.
Characteristic variations in photoconductors depending on the usage environment include, firstly, impairment of image characteristics in low-temperature, low-humidity environments. Ordinarily, the sensitivity characteristic and the like of the photoconductors drop apparently in low-temperature, low-humidity environments. As a result, worsened image quality becomes manifest in terms of lower image density and poorer gradation in halftone images. Image memory accompanying the worsening of the sensitivity characteristic may become likewise conspicuous. Image memory is an instance of image impairment wherein the image that is recorded in the form of a latent image, in a first drum rotation, is affected by the variation in potential in second and subsequent drum rotations, such that unwanted portions are printed, particularly during printing of halftone images. In particular, negative memory, where shading of a printing image is reversed, becomes often conspicuously observable in low-temperature, low-humidity environments.
Image characteristic deterioration in high-temperature, high-humidity environments is a further issue. In high-temperature, high-humidity environments, the moving speed of charge in a photosensitive layer is ordinarily greater than that at normal temperature and humidity. This gives rise to an excessive increase in print density, and defects such as small black dots (fogging) or the like to appear on white solid images. An excessive increase in print density translates into greater toner consumption, while the increased one-dot diameter may upset fine gradation. As regards image memory, a frequently encountered occurrence is positive memory wherein shading in a printing image remains reflected without changes, contrary to what is the case in low-temperature, low-humidity environments.
The underlying causes for characteristic deterioration depending on temperature and humidity conditions include, in many instances, moisture absorption and moisture release by the resin binder in the surface layer of the photosensitive layer, and by the charge generation material. Against this background, various materials have been studied, as in Patent Document 11 and Patent Document 12, where a specific compound is added to a charge generation layer, and, as in Patent Document 13, where a specific polycarbonate-based polymeric charge transport substance is used in a surface layer. However, no materials have been found as yet that succeed in fully satisfying the various characteristics involved in curbing the influence that temperature and humidity conditions exerts on photoconductors.
The technology disclosed in Patent Document 14 managed to solve the problem of characteristic deterioration derived from the abovementioned temperature and humidity conditions, but was not necessarily adequate as regards wear resistance. Patent Document 15, which discloses diallyl adamantanedicarboxylate that is used as a starting material of a resin that can be used as an optical material or electric material, failed to fully assess compounds having an adamantane structure as additive materials for photoconductors. Patent Document 16 discloses a photoresist composition that contains a compound having an adamantane structure, and Patent Document 17 discloses a resist composition that contains at least one type of a compound that has two or more adamantyl skeletons in the molecule. Patent Document 18 discloses carboxylic acid derivatives having an adamantane structure, and Patent Document 19 discloses a novel adamantane carboxylate compound. However, these documents failed to sufficiently assess the use of such compounds as additive materials for photoconductors. Patent Document 20 discloses an electrophotographic photoconductor that contains a polymer compound having a specific adamantane structure in a photosensitive layer, and Patent Document 21 discloses an electrophotographic photoconductor provided with a photosensitive layer that contains a specific adamantane-based compound. However, these photoconductors were likewise insufficient.    Patent Document 1: Japanese Patent Application Publication No. S57-122444    Patent Document 2: Japanese Patent Application Publication No. S63-18355    Patent Document 3: Japanese Patent Application Publication No. 2002-268250    Patent Document 4: Japanese Patent Application Publication No. 2002-287388    Patent Document 5: Japanese Patent Application Publication No. H6-75394    Patent Document 6: Japanese Patent Application Publication No. 2004-199051    Patent Document 7: Japanese Patent Application Publication No. 2004-206109    Patent Document 8: Japanese Patent Application Publication No. H08-272126    Patent Document 9: Japanese Patent Application Publication No. H11-288113    Patent Document 10: Japanese Patent Application Publication No. 2004-226637    Patent Document 11: Japanese Patent Application Publication No. H6-118678    Patent Document 12: Japanese Patent Application Publication No. H7-168381    Patent Document 13: Japanese Patent Application Publication No. 2001-13708    Patent Document 14: Japanese Patent Application Publication No. 2007-279446    Patent Document 15: Japanese Patent Application Publication No. S60-100537    Patent Document 16: Japanese Patent Application Publication No. H9-265177    Patent Document 17: Japanese Patent Application Publication No. 2002-55450    Patent Document 18: Japanese Patent Application Publication No. 2001-39928    Patent Document 19: Japanese Patent Application Publication No. 2003-306469    Patent Document 20: Japanese Patent Application Publication No. H4-174859    Patent Document 21: Japanese Patent Application Publication No. H6-161125
As described above, various conventional technologies have been proposed regarding improvement of photoconductors. However, the technologies disclosed in the patent documents above failed to sufficiently suppress adverse effects, on photoconductors, derived from harmful gas and the temperature and humidity environment, while satisfying sufficient wear resistance as well as various characteristics as a photoconductor. Further improvements were thus required.